Condenser microphones consist of two membranes: a membrane that is actuated by the sound pressure and a perforated membrane which forms a counter-electrode (“backplate”) that does not move in response to sound pressure, as the perforations render it acoustically transparent. The perforations allow the first membrane to move without pressure build-up in the volume between membrane and backplate.
In the presence of mechanical vibrations, also known as “body noise”, both plates are actuated. Due to differences in plate properties, the plates are actuated differently, so that the body noise results in relative movement of the plates. In the case of a microphone sensor, this relative movement is undesirably detected and, thus, reduces the sensor performance. In order to make the sensor intrinsically insensitive to body noise, the backplate must be designed such that it has the same response to mechanical vibrations as the membrane.
Both, sensing membrane and backplate are typically fabricated in tensile stressed layers. As the tension in the backplate might have an effect on the membrane properties or might lead to cracks, a method is needed to relax the stress in the backplate, so that the unwanted effect on the membrane is reduced.
Both stress relaxation and body-noise compensation can be achieved by attaching the backplate to elastic suspensions, such as springs. Spring-suspended (initially tensile-stressed) plates experience stress built-up at the anchors, as the plates are only attached at a limited number of sites along the plate's perimeter. This stress built-up decreases the robustness of these backplates, and means the design is sensitive to stress gradients. Also, when a spring-suspended plate is used, there is more backplate material removed near the rim of the plate than in the center. This might lead to processing problems associated with the resulting inhomogeneity of the sacrificial layer etch.
In conventional microphones with relatively large spacing between the two membranes, a few large holes in a rigid backplate are sufficient to reduce the effect of air damping. However, in miniature microphones that are built as micromechanical systems (MEMS) in Si-technology, the backplate is not rigid and should be perforated all over the plate to prevent air flow resistance, and this also enables it to be released by sacrificial layer etching.
It is noted that such perforations are also used in other moving or static MEMS-structures such as MEMS pressure sensors, MEMS electroacoustic transducers, MEMS switches, MEMS variable capacitors, or filters for gases or liquids.
Typically, a periodic pattern of square or circular holes is used, although rectangular holes have been proposed, in a more intricate pattern, for example as reported in M. Goto et al., High-performance condenser microphone with single-crystalline Si diaphragm and backplate, IEEE Sensors Journal 7, p. 4 (2007). A translational-periodic pattern of holes has the advantage that stress-gradients in the direction of the membrane normal (perpendicular to the membrane or in a vertical direction of the cross-section) do not cause a deflection of the two counter electrodes towards or away from each other. Any deflection of the backplate changes the equilibrium capacitance of the microphone.
FIG. 1 shows a top-view and cross-section through a (MEMS) condenser microphone.
The Si substrate 101 has an opening which exposes a part of the movable membrane 103 which is sensitive to acoustic pressure. The movable membrane is formed over an (optional) insulator 102. The backplate 105 (a fixed membrane) is suspended over a further insulator 104 and is perforated with a regular pattern of holes. The electrode connections 110,111 are to the two membranes and are used to measure the capacitance.
An issue in the fabrication of MEMS devices is that the stress and the stress gradient are difficult to control in processing.
One way to influence the stress is by altering the shapes and orientations of the perforations, and thereby varying the perforation degree. However, the resonance frequency is also lowered if the mass of membrane is decreased (by increasing the overall area of the perforations). If frequency matching is desired, this phenomena can be a sizable obstacle. At very high perforation degrees, manufacturing becomes difficult, the perforated membrane becomes fragile, and the capacitance and signal strength decrease. Typically, the smallest dimension for the hole diameter is 0.25-0.5 μm and 10-20 μm width between two openings.
Another problem is that of the mechanical toughness of perforated membranes. The holes lead to stress concentration around the edges which give rise to easier crack formation. Rounded edges are beneficial, but a circular shape is not the optimum in terms of electro-mechanical signal and noise generation.
This invention is based on a design to provide improved stress relaxation and/or improved mechanical toughness without the drawbacks discussed above.
US 2005/0229704 discloses an accelerometer with in-plane movement of a membrane. The membrane has openings to facilitate an etch release, for example a regular array of hexagonal openings.
US2011/075866 discloses a microphone with a backplate having an array of openings.